Year of Award

2021

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Geosciences

Other Degree Name/Area of Focus

Geophysics

Department or School/College

Department of Geosciences

Committee Chair

Rebecca Bendick

Commitee Members

W. Payton Gardner, Jesse Johnson, Hilary Martens, Sabrina Metzger

Keywords

earth deformation, geodesy, loading, seismicity, tectonics

Publisher

University of Montana

Abstract

Earth’s surface is constantly being stressed and deformed by countless dynamic processes that operate across a vast number of time and spatial scales. In this dissertation, I use geodetic observations in combination with time series analysis, basic numerical modeling, and a number of statistical tools to investigate the sources loading and observable deformation of earth’s surface and their relation to seismic activity.

In Central Asia, compressive stresses resulting from the collision of the Indian subcontinent and Eurasia have created a tectonically complex region that includes the best present-day example of ongoing subduction of continental lithosphere. Here, continental Eurasia is actively underthrusting the northern edge of the Pamir. Crustal faults that accommodate significant modern-day deformation can be linked to the southward dipping portion of a geometrically complex S-shaped intermediate depth seismogenic zone. To the west, beneath the Hindu Kush, this seismogenic zone dips steeply to the north and has not been linked to any crustal structure. Due to this it is often associated with down going Indian material. Using GPS time series from Central Asia, I show that localized shortening is not present on any active India- Hindu Kush crustal boundary, and that crustal convergence between India and Eurasia in Central Asia is absorbed primarily on the northern and western margins of the Pamir. This is consistent with one geometrically complex interface between subducting Asian lithosphere and the Pamir. This interface might curve westward such that the Hindu Kush seismic zone is a continuation of the Pamir seismic zone, or Hindu Kush earthquakes may occur in convectively unstable mantle lithosphere mechanically detached from surface faults.

Hydrologic processes have been shown to influence seismic productivity in many regions around the world, especially on active plate boundaries where tectonic forces are sufficient to critically stress the crust. To examine the influence of seasonal hydrologic loading on seismicity in intraplate regions and probe the regional crustal state of stress, we investigate temporal patterns of seismic productivity in the northern Rocky Mountains of Montana and Idaho. This is combined with analysis of GPS and SNOTEL time series. We find that temporal patterns in seismicity exist, with elevated productivity in December and January, and reduced productivity in June and July. We also find that seismicity is temporally correlated with the highest hydrologic loading rates as opposed to peak load, consistent with rate and state models of fault behavior for faults in critically stressed domains. However, we cannot distinguish between high hydrologic stress rates or pore pressure increases at seismogenic depths (~6 to 12 km) lagging ~6 months after peak snowmelt.

A feature universal to earthquake catalogs is the presence of independent and dependent events, that mainshocks, aftershocks, and foreshocks. Many studies of seismicity require the determination of dependent independent events (i.e. declustering) for investigations of background seismicity, earthquake hazards, and earthquake cluster analysis. While many declustering algorithms exist, choice of algorithm is often arbitrary. To address this issue, I compare the results of the four most commonly used declustering algorithms in four geologically distinct regions, determining the optimal method to be used in a number of situations.

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© Copyright 2021 Mason Perry